The Prisoner of Chillon: A Fable
How we cite our quotes:
For I had buried one and all
Who loved me in a human shape; (lines 320-1)
The speaker is completely alone in the world – whether in prison or out of it, he has no family or loved ones left alive.
The whole earth would henceforth be
A wider prison unto me: (lines 322-3)
Because he's completely alone, the speaker thinks that the whole world would just be a bigger prison. So what's the point of ending his imprisonment? He'd feel just as exiled and alone in the wider world.
A small green isle, it seem'd no more,
Scarce broader than my dungeon floor,
But in it there were three tall trees, (lines 344-6)
The small island the speaker sees in the middle of Lake Geneva can be seen as a double of his own prison cell: it's the same size as his "dungeon floor," and the three trees that grow on it could represent himself and his two brothers.